Refractor telescopes having air-spaced doublet objectives perform well with an f/5 relative aperture when used for low power purposes. However, at high power, they are limited by zonal spherical aberration and secondary chromatic aberration. The latter results in a purple fringe due to the fact that the common blue-red focus does not coincide with the yellow-green focus. These aberrations encountered at high power usage can be minimized by providing a relatively small relative aperture, e.g. f/10, or by using very expensive glass types for the lens elements.